Download 1 22,25 October 2004 Physiology of Locomotion R. B. Huey I. Some

22,25 October 2004
Physiology of Locomotion
R. B. Huey
I. Some Basic Issues to Explain
stamina
cost (ATP/time)
A. Here are some basic patterns: (i) the “cost” of locomotion (that is, ATP
consumed per time) increases with speed, (ii) stamina decreases with speed,
speed
record speed
stamina
speed
endotherms
ectothermms
distance
body mass
(iii) the world record speed declines as the distance of a race increases.
B. Also, ectotherms and endotherms have similar “burst” (or “sprint”) capacities,
but ectotherms have much less stamina. Thus most ectotherms are poor
marathon runners.
II. Why is locomotion energetically expensive?
A. Energy must be used to overcome inertia and thus move a body. Note that
muscles contract at high rates (1 Hz (Hertz) = 1 cycle/second)
mouse
7.5Hz
blow fly
120
elephant
l.4
mosquito
600
budgerigar
14
lizard
16
rattlesnake tail
100
B. Energy must be used to overcome frictional drag from the environment
1. For human at moderate speed, 7.5% total cost for friction
2. For sprinter, 13% (recall drag increases as velocity squared)
3. For birds, which fly at high speed, 25% cost goes to overcome drag
4. Drag can be much higher in water than in air, especially for a small animal,
which can find water to be very “viscous.”
C. Terrestrial animals must use energy to overcome gravity (aquatic animals are
essentially neutrally buoyant).
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D.  Other physiological “side-effects” -- increased metabolism caused by
activity will influence food needs, thermoregulation, water balance, circulation,
etc. Why?
III. Where do animals obtain energy for locomotion?
A. Muscle contraction takes considerable energy. Human muscles consume only
15% of animal's total metabolic rate at rest, but 90% during maximal activity.
B. ATP is the fuel for all muscular contraction. At the muscle level, ATP
utilization rates can increase 1000 fold during locomotion! A dynamic shift in
usage.
C. So, where does the ATP come from?
l. From existing muscle stores (= a battery)? No, very small amounts of ATP
are stored in muscle -- only enough for a few twitches during initial
activity.
2. Is it then synthesized de novo? Yes, via two pathways: a) Anaerobic
metabolism (pathways not using oxygen, see also below) or b) Aerobic
metabolism (pathways using oxygen) or both.
3. Which of these pathways is used depends on level of activity, and the time
course of the activity (burst vs. sustained)
glycogen
anaerobic
lactic acid + ATP
pyruvate
O2
Krebs cycle
aerobic
ATP, H20, CO2
4. If O2 supplies sufficient, the reaction via Krebs cycle (aerobic) is favored.
But if O2 supplies are not sufficient, pyruvate is instead converted to
lactic acid (anaerobic). So which pathway is used depends on O2
supply.
E. ATP used at the very beginning of activity is supplied from "phosphagens"
1. Creatine phosphate (PCr) is the phosphagen in vertebrates and also in
some invertebrates
PCr + ADP <--> ATP + Cr
CK
2. Creatine kinase (CK) is the enzyme that catalyzes this exchange
[3. Arginine phosphate (not PCr) is the phosphagen for most invertebrates]
4. No oxygen involved, so these are "anaerobic" pathways
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5. PCr provides ATP during initial stages of activity -- about 100 twitches.
[6. Note, however, that the resynthesis of PCr stores (following activity) of
PCr requires ATP, which comes from oxidative metabolism. So
ultimately oxygen is required.]
G. Energy source during low-level activity? Aerobic metabolism ("Pay as you
go")
1. Aerobic metabolism supplies energy at rest and during low-level activity
2. Fuels -- oxidation of carbon fuels (fats, carbohydrates, amino acids)
3. Very efficient -- 12 to 15 more ATP per mole glucose than anaerobic
glycolysis. Also relatively "clean" -- produces water and CO2.
3. The “Bad” -- requires O2. Only small intracellular stores of O2 are
available, and environmental sources (air, water) of new O2 are distant
from mitochondria in muscles. There is be a time lag to get new O2 to the
contracting muscle fibers -- maximal O2 consumption isn't reached for 1 to
2 min after exercise begins.
5. Moreover, aerobic metabolism has limited “scope” for expansion above
rest (below), such that it can generate only a limited amount of ATP.
6. Vertebrates increase aerobic metabolism (ATP production) by only 5 to
20X above rest, though some insects can achieve > 100X! ("Metabolic
scope" = maximum O2 - standard O2; "factorial scope" = maximum
O2/standard O2).
1 kg reptile
Stand O2
.1cc02/g.h
Max O2
.4
Metabolic Scope
.3
Factorial Scope 5.7
1 kg moth
1 kg bird
.1
14.6
14.4
121
.8
9.5
8.7
12
rate of ATP use
7. Why is limited scope a problem? A factorial scope of “10” means that the
maximum amount of ATP produced aerobically is only 10X that
produced at rest. If more ATP are needed (e.g., to sprint), then an animal
must obtain the extra ATP via anaerobic pathways.
H. Anaerobic glycolysis ("Twitch now, pay later")
1. As ATP needs begin to exceed aerobic capacity, anaerobic metabolism
begins to help supply ATP. The work load at which anaerobic metabolism
kicks in is called the “anaerobic threshold.”
“anaerobic
threshold”
anaerobic
aerobic
low
high
Speed
3 (or work load)
2. Fuel for anaerobic metabolism -- glycogen, stored as granules in skeletal
muscle cells
3. The “Good” -- because fuel is in the muscles and does not require O2,
anaerobic glycolysis delivers ATP very quickly. But intense activity can't be
sustained.
4. The “Bad” -- is inefficient – relatively few ATP fewer ATP per mole of
fuel.
5. Also "dirty" -- increase in lactate, a drop in pH, a temporary loss of
calcium from bone, and fatigue. In fact, high lactate suppresses further
glycolysis.
6. ⇒ Insects do not use anaerobic glycolysis (but apparently spiders do).
IV. “Oxygen debt”
A. After strenuous activity, animal continues breathing rapidly (thus, maintains
high level O2 consumption) for some time. Why is this necessary, if activity
has stopped? The “extra” O2 helps return the body to its original (preexercise) chemical state and is called (appropriately) the oxygen debt.
B. Primary functions of the oxygen debt
1. To resynthesize glycogen from the accumulated lactic acid (done mainly in
liver) that resulted from anaerobic activity (called "lactacid debt").
Requires O2.
2. To replenish creatine phosphate. Requires O2, creatine, ATP ("alactacid
debt").
C. Duration of oxygen debt is shorter in endotherms than in ectotherms, and is
shorter in small than in large animals. Why?
V. Putting it all together
A. Why is stamina inversely related to work load? Aerobic metabolism, which is
physiologically “clean” and efficient, can sustain modest work loads for very
long periods. But at higher work loads, ATP demands exceed the capacity of
aerobic pathways (“anaerobic threshold”) and anaerobic metabolism kicks in.
Although anaerobic pathways can produce lots of ATP quickly, they are nonsustainable (above). These reasons explain why stamina is much greater at low
work loads, and why the world record speeds are high only in short races.
B. Endotherms versus ectotherms.
1. All vertebrates use both aerobic and anaerobic metabolism, and both rely
on aerobic metabolism at rest.
2. Ectotherms sprint as fast as endotherms – anaerobic capacities similar.
3. However, a 1 kg iguana can support (aerobically) speeds of only 0.5 km/h,
a 1 kg rat can run aerobically at 4 km/h. Why do endotherms have greater
stamina than do ectotherms? This takes us back to “aerobic scope.”
4. Recall that aerobic output of ATP can increase by roughly 10X (= factorial
scope) in both vertebrate ectotherms and endotherms. But compare
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instead their “metabolic scopes,” which are proportional to the amount of
ATP that can be produced for above-rest needs. Aerobic scopes of
endotherms are much greater than those of ectotherms (see table above), so
can produce more “extra” ATP, which can be used for activity.
C. What work loads to “endurance workers” (e.g., lumberjacks, migrating animals)
sustain? Usually 4X rest, thus well below the maximum aerobic rates (which
are about 10X resting levels), but just below anaerobic threshold!
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